With the advent of substantial new uses for high bandwidth digital and analog electro-optic systems, there exists a greater need to effectively control the route and/or timing of electro-optic or optical signals from among many possible paths. This is especially true in digital computing systems where signals must be routed among processors; in analog systems such as phased array radar where both switching and time delay functions are fundamental; and in the switching of high bandwidth optical carriers in communication systems, However, it should be realized that these are just several of numerous electro-optic systems which require the use of optical switching, routing, interconnection, or time delay devices.
In many current and future systems light beams are modulated in a digital and/or analog fashion and used as "optical carriers" of information. There are many reasons why light beams or optical carriers are preferred in these applications. For example, as the data rate required of such channels increases, the high optical frequencies provide a tremendous improvement in available bandwidth over conventional electrical channels such as formed by wires and coaxial cables. In addition, the energy required to drive and carry high bandwidth signals can be reduced at optical frequencies. Further, optical channels, even those propagating in free space (outside of waveguides such as optical fibers) can be packed closely and even intersect in space with greatly reduced crosstalk between channels. Finally, operations that are difficult to perform directly in the lower (e.g., radio) frequencies, such as time shifting for phased array applications, can often be performed more efficiently and compactly using optical carriers.
A common problem encountered in many applications in which high data rate information is modulated on optical carrier beams is the switching of the optical carriers from among an array of channels. These differing optical channels may represent, for example, routes to different processors, receiver locations, or antenna element modules. One approach to accomplish this switching is to extract the information from the optical carrier, use conventional electronic switches, and then re-modulate the optical carrier in the desired channel. However from noise, space, and cost perspectives it is usually more desirable to directly switch the route of the optical carrier directly from the input channel to the desired channel. The required switching operations are typically either from a single input to among an array of outputs (one-to-n) for from among an array of inputs to an array of outputs (m-to-n).
In addition to the switching and routing function described above, control of the timing of optical signals is also very important in some applications--particularly in conjunction with phased arrays. Phased array systems are generally made up of arrays of many relatively isotropic radiators or emitters that are each driven coherently but with a relative phase (or time) delay among individual elements or among subarrays of elements. Controlling this timing across the array of radiators permits the array to form a beam that is strongly peaked in the far-field. Using this well established technique, the direction of the beam can be steered electronically (much faster than is possible mechanically) by controlling the phase shifts. In such applications it is highly desirable to provide time or phase shifting of signals for each emitter in a fast, accurate, compact, lightweight, inexpensive system while introducing minimal insertion losses and negligible spurious noise and crosstalk signals from scatter, reflections, and imperfect switch purity.
There are many practical barriers to implementing time delay networks directly in the microwave bands including large power splitters, bulky microwave cables or waveguides, high insertion losses, and dispersion. Photonic technologies can be applied to this problem, for example, by converting the microwave signals to modulation on optical carriers, introducing the required delays optically, and then converting back into the microwave regime. This type of translation scheme permits the use of optical devices and techniques in phase shifting that are superior to those operating directly in the microwave regime.
For Example, optical time delay networks can potentially be light weight, compact, and insensitive to electromagnetic crosstalk and interference. They can provide very long delays when required. Further, the dispersion effects are greatly reduced and multiple microwave bands can use the same delay network, Still, the advantages of using optical phase or time shifting must outweigh the overhead associated with converting to and from the optical regime.
Another technological challenge associated with driving phased array antennas arises in systems utilizing large bandwidth signals. When beam forming is accomplished by introducing phase delays (rather than time delays), large bandwidths cause the direction of the beam to detune from its desired direction. Since very large bandwidths are required for identification of targets and tracking, true time delay beam forming networks are also important in high performance phased array systems. In true time delay systems, time delays are introduced which correspond to phase delays larger than 2 pi, and the beam squint problem is reduced or eliminated.
For high performance in future phased array radar systems, it has been shown that time delay networks are needed with true time delay, low insertion loss, low crosstalk among the delay channels, and low spurious signal generation. Recently there has been much attention paid to the application of photonic time delay networks for addressing the phased array beam forming problem. There are many openings for the application of optical approaches in radar systems, and the concept of optical beam forming with time delay networks has been physically demonstrated [W. Ng, A. A. Walston, G. L. Tangonan, J. J. Lee, I. L. Newberg, and N. Bernstein, "The First Demonstration of an optically Steered Microwave Phased Array Antenna Using True-Time-Delay," Journal of Lightwave Technology, 9, 1124 (1991).] The bulk of the optical approaches, however, have been directed at switching and delaying the optical carrier using guided waves such as in optical fibers or planar waveguides. Many of these techniques use combinations of integrated optical switches and guided wave delay lines.
Other approaches for optically introducing time or phase shifts include the use of heterodyning and coherent techniques. Finally, free-space optics have been applied to phased array beam forming. These latter approaches have used segmented mirror spatial light modulators and polarization routing through prisms. However there is still much room for advancement in these approaches, particularly with respect to insertion losses, complexity, crosstalk, switch isolation, and multiple reflection suppression.
It is therefore an object of this invention to provide an optical switching and routing system that can independently route each and every optical carrier from an array in input channels to any of an array of output channels.
It is another object of this invention to provide optical switching, routing, and/or time delay systems that provide for a reduced complexity in terms of number of required optical switching elements and control points than other optical switches.
It is a further object of this invention to provide an optical switching and routing systems that exhibit a uniform delay for all possible switching or routing paths, and therefore introduce no relative skew in the switched signals.
It is a further object of this invention to provide optical switching and routing systems that provide a nearly lossless one-to-one optical interconnection from a set of input channels to a set of output channels.
It is a further object of this invention provide an optical time shifter system which has superior switch isolation, multiple reflection and crosstalk suppression; less complexity; lower insertion loss; and less stringent wavelength tolerances than time shifting systems of the past.
It is a further object of this invention to provide optical switching, routing, and/or time delay systems that incorporate a noise suppressor to enhance channel isolation of the switching system and reduce the level of crosstalk
It is still another object of this invention to provide an optical time shifter, switching, and/or routing system which is extremely compact.
It is still another object of this invention to provide optical time shifter, switching, and/or routing systems which provide wavelength insensitive performance.